Furyl radicals, reactions

The allylic-type furylic radical 6 is resonance stabilized to such a degree that its reactivity in promoting propagation by adding onto another furan ring is minimal. The fate of these radicals will simply be to couple with another radical present in the reaction medium (primary or secondary) or to disproportionate to regenerate the furan character of the ring26. ... 

The reactions of free radicals with furan and its derivatives can give both addition and substitution products depending on the specific system (11-13). With 2-substituted furans, the attack takes place predominantly at C5 and leads, by additon, to the corresponding furyl radicals which must be viewed as relatively stabilized interemediates because of the dienic-aromatic character of the furan heterocycle. These premises are essential to the understanding of the varied responses of furan monomers to free-radical activation. 

The fact that the typical furyl radicals in Scheme 6.6 are unable to propagate in the classical mode of free radical polymerization because of their resonance stabilization, has profound consequences in terms of the reactivity of furan monomers in this type of polymerization and the role of furan derivatives as possible perturbing agents in free radical chain reactions in general. These specific aspects will be illustrated below. 

Studies of cathodic reduction have been few. Amusingly, attempted anodic oxidation of the furyl ketone 123 actually resulted in cathodic reduction to the dimer 124 the corresponding ester was oxidized normally, however.301 Sometimes the dimethoxydihydrofurans formed in oxidation processes are reduced in a side reaction leading to the tetrahydrofuran derivatives.302 By using dimethylformamide as solvent instead of the protic solvents used formerly, a Czech group has demonstrated that the cathodic reductions of furans can produce fairly stable anion radicals having ESR spectra which agree well with theory.3023... 

Cyanofuran-2-[l,2,3,5-diselenadiazolyl] 30 was prepared similarly (Scheme 54) <2001IC6820>. The cyano-furyl-functionalized persilylated amidine 330, prepared by the reaction of 2,5-dicyanofuran with lithium bistrimethyl-silylamide, was treated with 2 equiv of SeCl2, which was prepared in situ from Se and SeCU- The diselenadiazolylium chloride [30] [Cl] was reduced with triphenylantimony to give the radical 30 in 68% yield from compound 330. 

According to this, the furan reacts with a radical-cation. Another Japanese group has suggested a more complex sequence to explain why benzene-1,4-dicarbonitrile comes to be attacked by furan in methanol with loss of one cyanide group. This reaction is promoted by phenathrene and on chromatography, the product loses the methanol to give 4(2-furyl)benzonitrile.18... 

A reaction without an oxygen parallel takes place when 2-furyl methyl thioether is treated with a Grignard reagent in the presence of a complex formed from nickel(II) chloride and a 1,3-propadiylbisphosphine derivative (Scheme 49). The sulfur is lost and the Grignard radical takes its place in reasonable yield (59%).204... 

In the above condensation resist designs, the phenolic resin offers a reaction site as well as base solubility. Self-condensation of polymeric furan derivatives has been utilized as an alternative crosslinking mechanism for aqueous base development (Fig. 126) [375]. The copolymer resist is based on poly[4-hydroxy-styrene-co-4-(3-furyl-3-hydroxypropyl)styrene], which was prepared by radical copolymerization of the acetyl-protected furan monomer with BOCST followed by base hydrolysis. The furan methanol residue, highly reactive toward electrophiles due to a mesomeric electron release from oxygen that facilitates the attack on the ring carbons, readily yields a stable carbocation upon acid treatment. Thus, the pendant furfuryl groups serve as both the latent electrophile and the nucleophile. Model reactions indicated that the furfuryl carbocation reacts more preferentially with the furan nucleus than the phenolic functionality. 

KH/18-crown-6/HMPA Scheme 4. Radical addition reaction of tri-2-furylgermane [Ge = (2-furyl)3Ge)... 

The ease with which this mechanism occurs has already been emphasized when discussing the side reactions associated with the cationic polymerization of 2-vinylfurans (Section 6.5.2) and with the self-condensation of fur-furyl alcohol (Section 6.6.2). However, its negative impact is also found, albeit with slower rates, in isolated furan polymers which possess the culprit C—H moiety as in the case of poly(2-vinylfuran) which slowly develops an insoluble fraction caused by the free radical mechanism shown in Scheme 6.28. These reactions are also responsible for the colour that is often associated with furan polymers. 

A radical generated from THF can be added across MA. Jacobs and Ecke obtained a 70% yield of the (tetrahydro-2-furyl)-succinic anhydride 70 during a peroxide-induced reaction of the THF with MA. 

When tetrahydro-2-methylfuran 71 was used, the product was (tetrahydro-2-methyl-2-furyl)-succinic anhydride 72. This suggests that the tertiary radical is the most favored species formed. The reactions were claimed to be specific for five-membered cyclic ethers since tetrahydropyran... 

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